Prior art devices for temperature regulation of a motor are known, which perform overload protection or current-limiting functions. Such devices have a measuring resistance through which the motor or load current passes, and whose voltage drop is detected by an evaluation circuit. When a predetermined limit value is exceeded, the current supplied to the motor is reduced or interrupted. Such circuits are described in application notes from semiconductor manufacturers, e.g. the Telefunken Databook 1984 "Phasenanschnittsteuerschaltungen und Nullspannungsschalter" (Phase Control Circuits and Zero Voltage Switches) or the Motorola/Plessey 1982 booklet, "Motor Speed Applications Using the TDA 2085A".
These prior art overload protection circuits have the disadvantage that, although the motor current does indeed provide information about the load state of the motor, it does not provide information about its temperature state. However, in the final analysis, only detection of heating of the motor provides adequate overload protection.
DE-OS No. 34 22 485 discloses a device in which the heating of the motor is estimated using a measuring resistance which is heated by the load current of the motor. The placement of the measuring resistance in the motor cooling air stream means that it measures not only the current-dependent heating, but is also influenced by the starting temperature of the cooling air. It is disadvantageous that, for translation of current values into heat values, a special structural form for the measuring resistance is necessary. It is further disadvantageous that the static quantities which determine the thermal time constants of the motor, namely heat resistance and heat volume or capacity, can be only very poorly simulated and varied in a measuring resistance.